The present invention relates to an optical line monitoring method for locating a failure point which has taken place in an optical fiber communication system or in an optical sensor system, and an apparatus therefor.
In a communication system, e.g., a telephone network, an optical fiber line (hereinafter simply referred to as "optical fiber") is occasionally used for a transmission line for connecting a central telephone exchange device with a terminal device. In such a large-scale optical communication system, the system configuring cost would be significantly high if many terminal devices are connected to the office device by optical fibers assigned to the individual terminal devices. To avoid this, a required number of one-input/multiple-output optical couplers/splitters are installed between each of terminal device groups and the office device, and the office device is connected with each optical coupler/splitter through a single optical fiber, for instance, thereby minimizing the cost for configuring the system.
In such an optical communication system, if disconnection or other trouble with the mechanical characteristic or transmission characteristic of an optical fiber occurs, then it is necessary to locate the failure point where the deterioration in the characteristic of the optical fiber has taken place so that proper corrective measures can be taken. For locating such a failure point, most offices are provided with optical pulse testing apparatuses such as OTDRs (optical time domain reflectometers), which operate based on the back-scattering method. A typical OTDR is designed to guide an optical output pulse from a semiconductor laser to an optical fiber to be tested and detect the back-scatter light which has been scattered from the optical fiber and has come back, thereby measuring the loss in the optical fiber.
The aforesaid optical communication system, however, incorporates optical couplers/splitters, and therefore, the use of the OTDR having a light source of a fixed wavelength does not make it possible to specify which of a plurality of optical fibers connected to the optical couplers/splitters has incurred a failure. For this reason, it cannot identify a failure point unless the failed optical fiber has already been found.
To solve the above-mentioned problem, in configuring an optical communication system, it has been suggested to provide the individual output ports of an optical coupler/splitter with band-pass filters which respond to transmission wavelengths or reflection wavelengths that are different from each other, and also to use an OTDR having a light source with a variable wavelength to locate a failure point. According to this suggestion, it is possible to specify an optical fiber incurring a failure and also to locate the failure point in the optical fiber. However, it is difficult to design a filter which always lets information communication light pass through and also properly reflects the monitoring test light. In addition, performing information communication and monitoring at the same time causes mutual interference.